First: While the PCIe slot may supply a total of 75W, only 5.5A may be drawn from 12V, so that maxes out at 66W. (I suppose you could use the 3.3V for things like RGB...)
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Second: The PCIe power connectors have a level of redundancy built in, so you can't assume you can just add multiple ones together:
6-pin has 2x +12V and 2x ground, but only needs one of each to supply the rated 75W. This means one wire/connector of each can be damaged and it will still meet any safety guidelines.
8-pin has 3x +12V and 3x ground; one extra of each. The number of "needed" wires doubled from the 6-pin, so the power rating also doubles.
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If the 12-pin were to have the same 1 extra wire (for both +12V and ground), that leaves 10 pins. It's likely there will be a couple of sense pins, so you'd be left with 4 pairs for power delivery (5 pairs connected of which 1 for redundancy) and you'd have 4x the capacity of the 6-pin, or 300W.